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United States Patent |
5,264,573
|
Ramert
,   et al.
|
November 23, 1993
|
Process for preparing 2-chloro-1,7-dihydropurin-6-one and a process for
its purification
Abstract
A process for the preparation of 2-chloro-1,7-dihydropurin-6-one which
comprises the steps of:
a) suspending 2-thioxanthine in concentrated hydrocloric acid, to produce a
suspension; and
b) contacting the suspension with chlorine, to produce
2-chloro-1,7-dihydropurin-6-one.
Inventors:
|
Ramert; Reiner (Weiler Bei Bingen, DE);
Christmann; Albrecht (Ingelheim am Rhein, DE)
|
Assignee:
|
Boehringer Ingelheim GmbH (Ingelheim am Rhein, DE)
|
Appl. No.:
|
729517 |
Filed:
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July 12, 1991 |
Foreign Application Priority Data
Current U.S. Class: |
544/265 |
Intern'l Class: |
C07D 473/40; C07B 037/00 |
Field of Search: |
544/265,264
|
References Cited
U.S. Patent Documents
2815346 | Dec., 1957 | Hitchings et al. | 544/264.
|
4405781 | Sep., 1983 | Bader et al. | 544/264.
|
Foreign Patent Documents |
001952 | Feb., 1966 | JP | 544/264.
|
Other References
Neiman Isr. J. Chem 3, 161 (1965).
Hurst, "Introduction to the Chemistry and Biochemistry of Pyrandines" pp.
64-81 (1980).
|
Primary Examiner: Berch; Mark L.
Attorney, Agent or Firm: Frankhouser; David E., Stempel; Alan R., Timbers; Mary-Ellen M.
Claims
What is claimed is:
1. A process for preparing 2-chlorohypoxanthine which comprises:
a) suspending 2-thioxanthine in concentrated hydrochloric acid, to produce
a first suspension; and
b) contacting the first suspension with chlorine, to produce
2-chlorohypoxanthine.
2. A process as recited in claim 1 wherein the first suspension is
contacted with the chlorine for about 3 to 7 hours.
3. A process as recited in claim 2 wherein the 2-chlorohypoxanthine
produced is produced as a precipitate in the form of 2-chlorohypoxanthine
hydrochloride monohydrate.
4. A process as recited in claim 3 which further comprises the steps of:
c) suspending the 2-chlorohypoxanthine hydrochloride monohydrate in an
aqueous solution of an alkali or alkaline earth metal hydroxide at a pH of
from about 6 to 7, to produce a second suspension;
d) heating the second suspension at a temperature of from about 50.degree.
C. to 70.degree. C.;
e) adjusting the pH of the suspension produced in b) to about 2 to 4; and
f) cooling the suspension produced in c) to about -5.degree. C. to
15.degree. C., to produce 2-chlorohypoxanthine as a precipitate.
5. A process as recited in claim 3 which further comprises the steps of:
c) suspending the 2-chlorohypoxanthine hydrochloride monohydrate in an
aqueous solution of an alkali or alkaline earth metal hydroxide at a pH of
about 9 to 10, to produce a third suspension; and
d) cooling the third suspension to about -2.degree. C. to 10.degree. C., to
produce a 2-chlorohypoxanthine hydrate as a precipitate.
6. A process for preparing 2-chlorohypoxanthine which comprises:
a) suspending 2-thioxanthine in concentrated hydrochloric acid, to produce
a first suspension;
b) contacting the first suspension with chlorine, to produce a first
reaction mixture;
c) concentrating the first reaction mixture by evaporation, to produce a
residue;
d) cooling the residue to about -5.degree. C. to 10.degree. C., to produce
2-chlorohypoxanthine hydrochloride monohydrate as a precipitate;
e) suspending the 2-chlorohypoxanthine hydrochloride monohydrate in an
aqueous solution of an alkali or alkaline earth metal hydroxide at a pH of
from about 6 to 7, to produce a second suspension;
f) heating the second suspension to a temperature of about 50.degree. C. to
70.degree. C.;
g) filtering the resultant suspension to produce an aqueous filtrate;
h) adjusting the pH of the aqueous filtrate to about 2 to 4;
i) cooling the adjusted aqueous filtrate produced in h) to about -5.degree.
C. to 10.degree. C., to produce 2-chlorohypoxanthine as a precipitate.
7. A process as recited in claim 6 wherein:
i) the 2-chlorohypoxanthine hydrochloride monohydrate is suspended in step
e) in an aqueous solution of an alkali metal hydroxide at a pH of from
about 6 to 7; and
ii) the second suspension is heated to a temperature of about 55.degree. C.
to 65.degree. C.
8. A process as recited in claim 6 wherein:
i) the 2-chlorohypoxanthine hydrochloride monohydrate is suspended in step
e) in an aqueous solution of sodium hydroxide at a pH of about 6.5;
ii) the second suspension is heated to a temperature of about 60.degree.
C.;
iii) the pH of the aqueous filtrate in step h) is adjusted to about 3; and
iv) the adjusted aqueous filtrate in step i) is cooled to about 5.degree.
C. to 10.degree. C.
9. A process for preparing 2-chlorohypoxanthine which comprises:
a) suspending 2-thioxanthine in concentrated hydrochloric acid, to produce
a first suspension;
b) contacting the first suspension with chlorine, to produce a first
reaction mixture;
c) concentrating the first reaction mixture by evaporation, to produce a
residue;
d) cooling the residue to about -5.degree. C. to 10.degree. C., to produce
2-chlorohypoxanthine hydrochloride monohydrate as a precipitate;
e) suspending the 2-chlorohypoxanthine hydrochloride monohydrate in a
aqueous solution of an inorganic base at a pH of about 9 to 10, to produce
a second suspension;
f) cooling the second suspension to about -2.degree. C. to 10.degree. C.,
to produce a 2-chlorohypoxanthine hydrate as a precipitate;
g) suspending the 2-chlorohypoxanthine hydrate in water, to produce a third
suspension;
h) adjusting the pH of the third suspension to about 6 to 7;
i) heating the adjusted third suspension produced in h) to a temperature of
about 55.degree. C. to 65.degree. C.;
j) filtering the resultant suspension to produce an aqueous filtrate;
k) adjusting the pH of the aqueous filtrate to about 2 to 4;
l) cooling the adjusted aqueous filtrate produced in k) to about -2.degree.
C. to 15.degree. C., to produce 2-chlorohypoxanthine as a precipitate.
10. A process as recited in claim 9 wherein the 2-chlorohypoxanthine
hydrochloride monohydrate is suspended in an aqueous solution of an alkali
metal hydroxide at a pH of about 9.5 to 10, to produce the second
suspension.
11. A process as recited in claim 9 wherein:
i) the 2-chlorohypoxanthine hydrochloride monohydrate is suspended in an
aqueous solution of sodium hydroxide at a pH of about 9.7 to produce the
second suspension;
ii) the second suspension is cooled to about 2.degree. C. to 5.degree. C.;
iii) the third suspension is adjusted to pH 9.7 with sodium hydroxide;
iv) the adjusted third suspension of step j) is heated to about 60.degree.
C.;
v) the aqueous filtrate is adjusted to pH 3; and
vi) the adjusted aqueous filtrate is cooled to about 5.degree. C. to
10.degree. C.
Description
The invention relates to a process for the preparation and purification of
2-chloro-1,7-dihydropurin-6-one (2-chloro-hypoxanthine).
##STR1##
2-chlorohypoxanthine (I) is derived from the base-structure 9H-purine
(which, when unsubstituted in the 7and 9-positions, is in equilibrium with
its tautomer 7H-purine [H. Beyer and W. Walter, Lehrbuch der organischen
Chemie, S. Hirzel Verlag, Stuttgart 1988, p. 797]).
In past years, derivatives of the purines have undergone a stormy period of
development in terms of their use as therapeutic agents [T. W. Stone Ed.,
Purines - Pharmacology and Physiological Roles, VCH Verlagsgesellschaft,
Weinheim 1985].
Thus, a large number of nucleosides having a purine partial structure have
antimetabolic properties. Some nucleoside derivatives have been found
which are capable of restricting the replication of the HIV virus
responsible for AIDS.
This group of effective nucleosides includes, in particular, nucleosides in
which the so-called nucleoside base has a naturally occurring or
derivatised purine system. The use of purine derivatives which are
derivatised at the nitrogen atom in the 9-position and functionalised in
the 2-position appear particularly promising [see also European Patent
Application 343 133 and 291 917]. 2-Chlorohypoxanthine (I) would appear to
be a suitable starting material for derivatised purines of this kind,
being already suitably functionalised in the 2-position.
The processes hitherto known from the literature for preparing this
attractive intermediate compound do, however, suffer from the defect that
industrial production is only possible at high cost, or that the necessary
educts are not commercially available.
Thus, J. A. Montgomery and L. B. Holum [J. Am. Chem. Soc. 79 (1957) 2185]
describe a method of synthesis starting from 2,6-dichloropurine. A similar
process is described by Y. Yamada et al. [Chemical Abstracts 66 (1967)
9536 8a]. The disadvantages of both methods are, in particular, the fact
that the partial hydrolysis of the dichloro compound requires considerable
dilution of the starting material and the 2-chlorohypoxanthine can only be
isolated in a yield of 66% (crude product). Furthermore,
2,6-dichloropurine is expensive, not commercial available in large
quantities and represents--because of its critical irritant, its
allergenic and potential cancerogenic, character--a substance, the
handling of which raises important problems with regard to safety
engineering and working hygiene. These disadvantageous aspects indicate
that this type of synthesis would be impractical for the industrial
production of 2-chlorohypoxanthine.
The second method described in the literature starts from
2,8-dichloro-6-hydroxypurine [E. Fischer, Chem. Ber. 30 (1897) 2208; ibid.
30 (1897) 2226] which is not, however, commercially available and
accordingly has first to be produced from uric acid and
phosphorusoxychloride.
The 2,8-dichloro-6-hydroxypurine synthesised in this way has to be
partially dehalogenated in another reaction step either with hydrogen
iodide or by selective catalytic hydrogenolysis [H. Ballweg, Liebigs Ann.
Chem. 649 (1961) 114]. Whereas dehalogenation with hydrogen iodide is very
complex and impractical for industrial application, in catalytic
hydrogenolysis the reaction product has to be recrystallised several times
from water and is then obtained in a yield of only 77%, which means that
this process is also unsuitable for adoption on an industrial scale.
In addition, very high standards have to be imposed on the purity criteria
of intermediate products used in manufacturing methods for synthesising
pharmaceuticals.
The aim of the present invention is to provide a process for preparing
2-chlorohypoxanthine (I) (2-chloro-1, 7-dihydropurin-6-one) which allows
this purine derivative to be produced easily and in good yields.
A further objective of the present invention is to provide a method of
synthesis in which the reaction products or intermediate products are
obtained in a crystalline form which permits trouble-free further
processing--centrifugation, drying, etc.--even of industrial-scale
batches.
A further aim of the present invention is to provide a method of
purification in which the 2-chlorohypoxanthine (I) is obtained in a degree
of purity which allows the product to be used directly as a starting
material in other processes for preparing pharmaceutically active
substances.
In its broadest aspect, the invention provides a process for preparing
2-chlorohypoxanthine, which may be obtained in the form of a salt or a
hydrate, the process comprising reacting 2-thioxanthine suspended in a
concentrated hydrochloride acid, with chlorine.
Preparation of the 2-chlorohypoxanthine-hydrochloride monohydrate
##STR2##
According to the invention, the objectives outlined above are achieved by
first preparing the monohydrate of 2-chlorohypoxanthine-hydrochloride
starting from commercial 2-thioxanthine by chlorination in the presence of
concentrated hydrochloric acid. To do this, the 2-thioxanthine is first
suspended in concentrated hydrochloric acid and chlorine is introduced
into the reaction mixture over a period of 3 to 7 hours--preferably about
5 hours--at a temperature of 0.degree. C. to 10.degree. C.--preferably
2.degree. C. to 7.degree. C. and especially 3.degree. C. to 5.degree. C.
Then some of the hydrochloric acid is distilled off and the reaction
mixture is cooled to a temperature in the range from -5.degree. C. to
+10.degree. C.--preferably 0.degree. C. to 5.degree. C.--to cause
precipitation of the required 2-chlorohypoxanthine in the form of the
monohydrate of its hydrochloric salt. The reaction mixture is then suction
filtered and the crystalline residue is washed first with saline solution
and then with an organic solvent, preferably an alcohol, especially
isopropanol, and dried.
Preparation of the 2-chlorohypoxanthine
##STR3##
In a subsequent reaction step the 2-chlorohypoxanthine hydrochloride
monohydrate is suspended in an aqueous solution at a pH ranging from 6 to
7 and, preferably, a pH of 6.5, which is adjusted with an aqueous solution
of an alkali or alkaline earth metal hydroxide, preferably an alkali metal
hydroxide and especially concentrated sodium hydroxide solution. After the
suspension is heated to a temperature in the range from 50.degree. C. to
70.degree. C.--preferably 55.degree. C. to 65.degree. C. and more
especially to a temperature of 60.degree. C. The resulting solution is, if
necessary, treated with decolorising charcoal and filtered. An aqueous
solution of an acid is admixed with the solution to adjust the pH to a
value in the range from 2 to 4 and, preferably, 3.0. The acid is
preferably an inorganic acid and especially hydrochloric acid.
The resulting solution is cooled to a temperature in the range from
-5.degree. C. to +15.degree. C.--preferably 5.degree. C. to 10.degree. C.
to precipitate 2-chloro-hypoxanthine. The precipitate is washed with water
and then with an organic solvent, preferably an alcohol, more particularly
isopropanol, and dried.
Preparation of sodium 2-chlorohypoxanthine hydrate
##STR4##
In order to prepare the sodium salt of 2-chlorohypoxanthine in highly pure
form, 2-chloro-hypoxanthine-hydrochloride-monohydrate is suspended in
water. Whilst cooling, the pH is adjusted to a value in the range from 9
to 10, preferably, 9.5 to 10, and more preferably, 9.7, using an aqueous
solution of an inorganic base, preferably a solution of an alkali metal
hydroxide and more preferably using concentrated sodium hydroxide
solution.
The reaction mixture is then cooled to a temperature below ambient
temperature, preferably to a temperature in the range from -2.degree. C.
to +10.degree. C., more preferably 2.degree. C. to 5.degree. C. to
precipitate the hydrate of the desired sodium salt. The crystalline
precipitate is suction filtered and washed first with cooled brine and
then with an organic solvent, preferably an alcohol and more especially
isopropanol, and dried.
Preparation of high purity 2-chlorohypoxanthine (I)
##STR5##
The pH of a suspension of sodium 2-chloro-hypoxanthine hydrate in water is
adjusted to a value in the range of 6.5, using acid, preferably an
inorganic acid and more especially dilute hydrochloric acid. The
suspension is then heated to a temperature in the range from 55.degree. C.
to 65.degree. C., preferably 60.degree. C.
The resulting solution is filtered, optionally after treatment with
decolorising charcoal. The pH of the filtrate is adjusted to a value in
the range from 2 to 4, preferably a pH of 3.0, using an aqueous solution
of an acid, preferably an inorganic acid and more particularly 1 N
hydrochloric acid. The resulting suspension is then cooled to a
temperature below ambient temperature, preferably to a temperature in the
range from -2.degree. C. to +15.degree. C., more especially from 5 to
10.degree. C., to precipitate the desired 2-chloro-hypoxanthine. The
resulting precipitate is suction filtered and washed with water and then
with an organic solvent, preferably an alcohol and more especially
isopropanol.
The crystalline residue is dried. The 2-chloro-hypoxanthine obtained in
this way has a purity of more than 99%, according to HPLC analysis.
The process steps explained above are described more accurately by the
reaction sequence mentioned in the Examples. Various alternative
embodiments of the process and the like will become apparent to anyone
skilled in the art from the description. However, it is expressly pointed
out that the Examples and the related specification are provided solely
for the purpose of explanation and description and should not be regarded
as a restriction of the invention.
EXAMPLES
1) Preparation of 2-chlorohypoxanthine-hydrochloride-monohydrate
230 g (3.24 mol) of chlorine are introduced into a suspension of 168.2 g
(1.0 mol) of 2-thioxanthine in 68 liters of conc. hydrochloric acid in the
course of about 5 hours at 3.degree.-5.degree. C. Then about 1 liter of
hydrochloric acid is distilled off in a water jet vacuum, the residue is
cooled to 0.degree.to +5.degree. C., suction filtered and washed with 0.5
liters of saturated, ice-cold brine and finally with 300 ml of
isopropanol. After drying at 50.degree. C. over a period of about 12
hours, 93 g (85.7% of theory) of colourless crystal powder are obtained.
By preparing the free base and again converting it into the hydrochloride
the title compound is obtained in analytically pure form. Elemental
analysis corresponds to the composition C.sub.5 H.sub.4 Cl.sub.2 N.sub.4
O.times.H.sub.2 O.
2) Preparation of 2-chlorohypoxanthine
180 g (0.80 mol) of 2-chlorohypoxanthine-hydrochloride-monohydrate are
suspended in 3.6 liters of water and the pH is adjusted to 6.5 by the
dropwise addition of concentrated sodium hydroxide solution. After heating
to 60.degree. C. the solution is treated with 18 g of decolorising
charcoal and filtered. The filtrate is added dropwise to 300 ml of water,
whilst a pH of 3.0 +/- 0.5 is maintained in the aqueous solution using 1N
hydrochloric acid. The resulting suspension is cooled to
5.degree.-10.degree. C., suction filtered and the precipitate is washed
with 0.5 liters of water at a temperature of +5.degree. C. and with 300 ml
of isopropanol. After drying at 50.degree. C., 117 g (85.8% of theory) of
the title compound are obtained in the form of a colourless crystal
powder. By purifying via the sodium salt and re-liberation of the base,
analytically pure material is obtained which is identical to the substance
prepared from 2,6-dichloropurine according to J. A. Montgomery and L. B.
Holum [J. Am. Chem. Soc. 79 (957) 2185].
3) Preparation of sodium 2-chlorohypoxanthine hydrate
80 g (0.3mol) of 2-chlorohypoxanthine-hydrochloride-monohydrate are
suspended in 1.6 liters of water. Whilst cooling, a pH of 9.7 is adjusted
using concentrated sodium hydroxide solution. After cooling to +5.degree.
C. the precipitate is suction filtered, then washed with 250 ml of 20%
brine (previously cooled to +5.degree. C.) and 250 ml of isopropanol.
After drying at 50.degree. C., 65.7 g (77.6% of theory) of the title
compound are obtained in the form of a colourless crystal powder. By
suspending in cold water and again suction filtering, washing with cold
water and methanol and drying, an analytically pure product is obtained,
the composition of which corresponds to the formula C.sub.5 H.sub.2
ClN.sub.4 ONa.times.2.5H.sub.2 O, according to elemental analysis.
4) Preparation of highly pure 2-chlorohypoxanthine
20 g of sodium 2-chlorohypoxanthine-hydrate are suspended in 400 ml of
water, the pH is adjusted to 6.5 in the reaction solution using dilute
hydrochloric acid and the mixture is heated to about 60.degree. C. After
treatment with 2 g of decolorising charcoal and filtration, the
2-chlorohypoxanthine is precipitated and isolated in the same way as
described in Example 2 (at pH 3.0 +/- 0.5 and at ambient temperature).
11.6 g (8% of theory) of the title compound are obtained in the form of
colourless crystals. According to HPLC analysis the product has a degree
of purity of more than 99%.
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